When information is recorded on a floppy disk at high density, the positioning of an information head device in the direction perpendicular to a track is required. In this case, if the positioning of the information head device is mechanically conducted, sufficient positioning accuracy cannot be secured. Therefore, recently, the positioning of the information head device has mainly been conducted by using light beam.
Hereinafter, a conventional optical head device in which the positioning of the information head device is conducted by using a light beam will be described with reference to FIG. 20.
As shown in FIG. 20, a light beam emitted from a semiconductor laser 101 serving as a light source is divided into a zero-order light beam and a .+-.first order diffracted light beam (.+-.first order diffracted light beam is not shown) by a diffraction grating 152 provided on the surface of a diffraction element 150 at the side of the light source. Hereinafter, the zero-order light beam will be referred to as "the main beam" and the .+-.first order diffracted light beam will be referred to as "the sub-beam".
The main beam and sub-beam pass through a diffraction grating 151 provided on the surface of the diffraction element 150 at the side of a lens 102 and are converged by the lens 102 serving as an converging optical system. The main beam and sub-beam converged by the lens 102 are limited to the desired numerical aperture NA by an aperture stop 103 and irradiate a disk 104 serving as an information recording medium. On the disk 104, a line connecting the main beam spot and two sub-beam spots is arranged so as to have a predetermined angle with respect to a track. Moreover, the disk 104 is sandwiched by magnetic heads 201 being attached to an arm 211 and serving as the information head device, and thus the position of the disk 104 in the direction of the z-axis is regulated. Furthermore, each member of the optical system and the magnetic heads 201 are fixed in a frame 210. Thus, the distance between the optical system and the disk 104 always remains constant (20 mm in this case).
The light beam reflected from the disk 104 passes through the aperture stop 103 and the lens 102 again, is diffracted at the diffraction grating 151 provided on the surface of the diffraction element 150 at the side of the lens 102, and then enters the photo detectors 105R and 105L.
Each of the photo detector 105R and 105L consists of three detecting regions respectively, receives the main beam and two sub-beams separately and outputs signal in accordance with the quantity of the received light.
On the disk 104, three beam spots irradiate the different positions in the direction perpendicular to the track. Therefore, the modulation degrees of signals obtained by the three detecting regions are different from each other. The modulation degree of signal is sequentially changed as these beam spots cross the track. Therefore, by calculating these signals, the relative position relationship between the track and the beam irradiation position can be detected.
Namely, first, an envelope detection of these signals is conducted and the obtained envelope signals are defined as M, S1 and S2. Next, by using these envelope signals M, S1 and S2, the calculation of A=M-S1 and B=S2-M are conducted so as to generate signals A and B. By multiplying the signals A and B by an appropriate factor k1, tracking error signals k1.multidot.A and k1.multidot.B, which zero-cross in optional phases, are obtained. By using these tracking error signals k1.multidot.A and k1.multidot.B, the relative position relationship between the track and the beam irradiation position can be detected. Herein, the factor k1 is initially learned so that the magnetic head 201 is positioned on the track for recording and reproducing information. Moreover, both the magnetic head 201 and the optical system move in the radius direction of the disk 104 by a moving means 203 shown in FIG. 7.
In the above-mentioned conventional optical head device, the lens 102 having a focal length f of 10 mm is used. When the optical system of single magnification is constructed as in FIG. 20, the distance between the semiconductor laser 101 serving as the light source and a light converging point of the disk 104 serving as an information recording medium (the distance between an object point and an image point) is about 40 mm, and the distance L2 between the lens 102 and a geometrical optical converging point is 20 mm. Furthermore, when the numerical aperture NA is 0.04, the radius a of the opening in which the aperture stop 103 having the opening of radius a' is projected on the principal plane of the disk side of the lens 102 is 800 .mu.m. Furthermore, when the wavelength .lambda. of the light beam emitted from the semiconductor laser 101 serving as the light source is 800 nm, a spot diameter d=.lambda./NA on the disk 104 is 20 .mu.m, and the Fresnel number N=(a.times.a)/(.lambda..times.L2) is 40. In addition, the positional difference between the geometrical optical converging point and a point in which the intensity of the converged light is maximum is about 20 .mu.m. Since the focal depth .DELTA.z that is a distance between the point in which the intensity of the converged light is maximum and the point in which the intensity of the converged light is 80% of the maximum is about 80 .mu.m, this positional difference is substantially negligible.
However, in the above-mentioned conventional optical head device, it is necessary to increase the distance between the object point and the image point so that the positional difference between the geometrical optical converging point and the point in which the intensity of the converged light is maximum is substantially negligible. Accordingly, it is impossible to miniaturize the optical head device. Furthermore, when the distance between the object point and the image point is longer, the light beam emitted from the light source of the optical head device is susceptible to environmental changes such as change of temperature or vibration. Thus, a stable positioning of the information head device cannot be conducted.